A constantly growing number of information systems and environmental influences deliver demanded and non-demanded information to people. In this respect, a mobile presentation of information has increasing importance. Microdisplays, i.e. very small displays with picture diagonals of less than or equal to 20 mm, in this respect offer the possibility of representing photographic and video information in high resolution and in a user-specific manner, i.e. for only one user or for a plurality of users. Areas of application of microdisplays can be seen in the field of near-to-eye applications. They include, for example, video glasses which can be connected to mobile multimedia devices (smartphones or mobile audio and video players). These video glasses can be used for mobile TV, video presentation or presentation of internet content. Furthermore, microdisplays can be used in digital cameras and/or video cameras as high-resolution electronic viewfinders.
A further area of application is augmented reality. The microdisplay is mounted in see-through optics (glasses) for these applications. The user sees the real environment through these glasses and additional information in the form of images, texts, graphics, etc. can be superimposed on this image via the microdisplay. This can be utilized, for example, in the servicing of complex plant and machinery for fading in assembly instructions or general instructions. In aeronautical engineering, pilots can have the display of different measurement instruments added. In medicine, the data from important devices can additionally be presented for surgeons. Furthermore, a variety of applications are conceivable in the military sector.
Further applications of microdisplays are pico projectors, i.e. very small projectors which project image and video content onto a planar surface and present it visible to a plurality of users. Such projectors with microdisplays can also be used in metrology for projecting defined patterns onto a surface to be examined and for the subsequent optical detection of the 3D structure of this surface.
Very high brightness values (>10000 Cd/m2) are in particular required for projection applications and see-through applications. In contrast to this, comparatively low brightness values (≦150 Cd/m2) are required for the multimedia applications and video glasses. There should be the possibility of addressing all these applications with one display for microdisplays on the basis of organic light-emitting diodes (OLEDs). In this respect, high-resolution image information having a brightness adjustable over several orders of magnitude (from <100 Cd/m2 to above 10000 Cd/m2) should be able to be presented. An extension of the electric current and voltage range which such a circuit has to be able to control is required for this purpose.
Currently, different microdisplay technologies are available. In this respect, it is possible to distinguish between light-modulating (non-emitting) technologies and light-emitting technologies.
The light-modulating displays include LCOS (liquid crystal on silicon) and MOEMS based microdisplays. These technologies demand an additional external illumination which increases the complexity, the size and the weight of the overall system, but simultaneously only provide limited contrast (typically <1:100).
Innovative self-emitting flat displays having many advantages can be implemented on the basis of organic light-emitting diodes (OLEDs). They include the possible large-area deposition, the self-lighting properties which allow very thin and low-power displays and the potentially high efficiency of such displays. OLED microdisplays are currently equipped with monochrome or broadband (white) emitters. For color OLED microdisplays, the primary display colors are frequently realized by a white emitter and the additional application of a color filter.
All the named technologies are formed with active and passive components (transistors and capacitors). Every organic light-emitting diode as a pixel (picture element) is controlled in this respect by its own integrated electronic circuit. This pixel circuit is designed in this respect so that it can be written with the image information in the form of an electric voltage or of a current. The image information is stored in the circuit associated with the organic light-emitting diode and this circuit drives the OLED with an electric current or with a voltage which corresponds to the stored image information.
Currently the following concepts have been realized in this respect:    1. Programming the respective circuit of an organic light-emitting diode with an analog electric current whose magnitude is proportional to the gray-scale value of the image information to be presented. This analog electric current is converted into an analog voltage and is stored by means of a capacitor. The stored electric voltage is converted into an electric current corresponding to the image information. This current influences the respective organic light-emitting diode. The brightness is in this respect set by the magnitude of the electric current which flows through the organic light-emitting diode (analog value). Gray-scale values/gradations of the brightness are realized by a correspondingly smaller electric current.    2. An electric voltage can be stored in a capacitor. The electric voltage is in this respect converted into an electric current in the circuit associated with the respective organic light-emitting diode. This current influences the brightness at which electromagnetic radiation is emitted by the organic light-emitting diode. The brightness is in this respect set by the magnitude of the electric current which flows through the organic light-emitting diode (analog value). The gray-scale value is in this respect realized as under 1.).    3. A programming of the circuit for an organic light-emitting diode can be achieved using an analog electric voltage and storing the voltage on a capacitor. The organic light-emitting diode can be operated using the stored electric voltage or a voltage whose magnitude corresponds to this stored voltage. The brightness is set by the magnitude of the electric voltage applied to the organic light-emitting diode. Gray-scale value/gradations can be realized by a correspondingly smaller electric voltage.    4. The programming of the circuit of organic light-emitting diodes can take place using digital electric voltages and storing these digital voltages/states on capacitors. The number of capacitors corresponds to the bit width of the image information for a pixel (usually 5, 6 or 8 bits). The organic light-emitting diode is controlled by a time-pulsed electric current of constant magnitude. The number of pulses per image sequence in this respect corresponds to the bit width of the image information. The length of the pulses is in this respect dependent on the value of the bits. Depending on the digital state of the individual storage capacitors of a circuit associated with an organic light-emitting diode, the electric current through the organic light-emitting diode is switched on or off for the corresponding pulse duration. The brightness of the emitted radiation can be set by the magnitude of the electric current flowing through the organic light-emitting diode. Gray-scale values/gradations are influenced by pulse width modulation of the electric current. The dynamic range of the electric current flowing through the organic light-emitting diode and the maximum voltage drop over the OLED are limited in this respect.            The field of use of microdisplays having organic light-emitting diodes is restricted to low (≦200 Cd/m2) to medium brightness values (up to 5000 Cd/m2), i.e. to the areas of application with the information presentation for an individual person and near-to-eye applications. The field of use is limited by the maximum presentable brightness of such microdisplays. The brightness depends on the efficiency and on the voltage requirements of the organic light-emitting diodes and the electric current and voltage driver capability of the circuit.        
No solutions are known in which microdisplays having organic light-emitting diodes are utilized for projection applications and see-through applications with high maximum brightness values (≧10,000 Cd/m2) and are controlled using corresponding circuits associated with the organic light-emitting diodes.
The area of use of the currently available OLED microdisplays is restricted to unidirectional, image-reproducing microdisplays. In accordance with DE 10 2006 030 541 A1, a use can also be realized in bidirectional microdisplays, i.e. microdisplays having an image presentation function and an image taking function or optical detection function.